The present invention pertains to the field of polymer blend morphology stabilization. Specifically, the invention uses a filler material to modify the viscosity of a lower viscosity dispersed phase in a matrix of an immiscible higher viscosity polymer, resulting in a stable morphology of the blend.
It is well known in the art of polymer blend development that the equilibrium morphology in a blend system is a balance between drop breakup and coalescence of the dispersed phase. The tendency for drops of the minor phase to break up depends on the ratio of the maximum shear stress that deforms the drop to the interfacial stress that resists the breakup. It is affected by the interfacial tension between the components of the blend, differences in the viscosity of the dispersed phase and the matrix phase, and the shear forces to which the blend is exposed during processing.
Most coalescence studies in polymer blends have not used mechanical mixing. Rather, researchers have studied coalescence in solvent cast blends or melt blends prepared under quiescent conditions. Coalescence in molten blends without the influence of mechanical stress has been modeled by Fortelny and Kovar (I. Fortenly and J. Kovar, J. Polym. Comp., 9, 119 (1988)). They found that the amount of coalescence in blends decreases significantly if the matrix phase viscosity is above a critical value. Other researchers have shown that the coalescence depends on the probability of collisions and that the probability increases with increasing volume concentration of the dispersed phase. It has also been proposed that interfacial compatibilization reduces the interfacial mobility, hence reducing the probability for coalescence (S. Endo, K. Min, J. L. White and T. Kyu, Polym. Eng. Sci., 26, 45 (1986); R. Fayt, R. Jerome and P. Tessie, Makromol. Chem., 187, 837 (1986); B. D. Favis and J. P. Chalifoux, Polymer, 29, 1761 (1988); A. Nakayama, P. Guegan, A. Hirao, T. Inoue and C. W. Macosko, ACS Polymer Preprints, 34 (2), 840 (August 1993)).
Coalescence is a function of the volume fraction of the dispersed phase, drop size, the mobility of the interface, and the mixing conditions. For Newtonian fluids, coalescence of a dispersed phase has been explained by models which account for the probability of particle collisions and the efficiency of such collisions. For non-Newtonian fluids, the probability of diffusion related collisions is overcome by the much greater probability of collisions due to shear forces, particularly under molding conditions, and most especially under abusive injection molding conditions where shear forces are high. Coalescence of the particles of the dispersed phase results in their uneven distribution. Therefore, as a result of the high shear forces experienced during injection molding operations, the minor phase often coalesces. When molded parts are made from blends where coalescence of the minor phase is a problem, the molded parts suffer from severe delamination and have a poor surface appearance as well as reduced ductility.
In the past, stabilization and prevention of coalescence have often been achieved by the addition or the formation of block or graft copolymers at the interface between the phases. These copolymers join at least a fraction of the two phases by covalent bonds, lowering interfacial tension and retarding coalescence. Retardation of coalescence in the blend improves the stability of the blend morphology and thereby reduces the deleterious effects that processing history can have on the performance of the blend, such as delamination. The failure properties of the blend also are improved when the desired morphology is stabilized by the proper degree of interfacial adhesion.
Addition of block copolymers or the use of functionalized homopolymers which can react to form copolymers in situ (xe2x80x9creactive compatibilizationxe2x80x9d) is an effective method for compatibilization of two immiscible phases in a polymer blend and prevention of coalescence. However, the block copolymers (or other copolymer-like core shell polymers) and functionalized homopolymers are expensive to produce. Moreover, in situ functionalization of homopolymers by adding functionalizing agents during the process of extrusion can result in some instances in the formation of undesirable side products or in a reduction in the polymer molecular weight. These side reactions can detrimentally affect other blend properties.
U.S. patent application Ser. No. 09/293,915, filed Apr. 19, 1999 is concerned with a method for improving the physical properties of thermoplastic molding compositions. A polyester or polyamide is blended with an impact modifier and an epoxy- or orthoester-functionalized compound. The epoxy- or orthoester-functionalized compound is the compatibilizing agent for the polyester or polyamide blend and an impact modifier. The impact modifier consists of several silicone components such as a polysiloxane compound, an inorganic filler, and optionally, a silicone additive. The silicone powder or silicone rubber combined with the compatibilizer yields a blend product with high impact strength. One objective of this invention is to improve the physical properties of polyester- or polyamide-based blends through the combination of fillers in the dispersed phase and the compatibilization through an epoxy functionalized fluid.
U.S. Pat. No. 5,102,941 is directed to a thermoplastic polyester composition composed of a polyester, a crosslinked polyorganosiloxane latex rubber, a functionalized silane containing at least one alkoxy or chloride radical, and optionally, a filler. The polyorganosiloxane rubber is a crosslinked latex with an average particle diameter of 0.1 to 0.5 xcexcm and a swelling degree of 3 to 50. The functional groups on the silane may be epoxy, including for example glycidoxyalkyl or xcex2-(3,4-epoxycyclohexyl)ethyl, isocyanate, or amino groups. Reinforcing fillers which optionally may be added to the blend are glass fibers, carbon fibers, aramid fibers, metal fibers, asbestos fibers, whiskers, glass beads, glass flakes, calcium carbonate, talc, mica, aluminum oxide, magnesium hydroxide, boron nitride, beryllium oxide, calcium silicate, clay, and metal powders. The addition of silica fillers is not suggested. In addition, the dispersed rubbery phase is crosslinked.
The use of fillers to compatabilize polymer blends has been shown by Rodrigues and Ishida (Macromol. Symp. 104, 89-111 (1996)), where specific chemical treatments on the surface of the filler have been used to compatabilize two imiscible blends. The method described by Rodrigues and Ishida involves melt blending a surface treated glass bead filler with a polycarbonate/polypropylene system to form thermoplastic pellets. The method is hereinafter known as xe2x80x9cProcess Bxe2x80x9d. It has been observed that in the case with the right chemical treatment, the filler is located at the interface between the two polymers and compatabilizes the blends.
U.S. Pat. No. 5,391,594 is directed to a method for imparting fire retardancy to organic resins through the addition of a silicone polymer powder. The method involves mixing a silica filler with a polydiorganosiloxane polymer to form a powder. The silica filler used has an average particle size of 1 to 1000 microns. The resulting powder is then melt blended with a thermoplastic resin to form thermoplastic pellets. The method described in this patent is hereinafter known as xe2x80x9cProcess Axe2x80x9d.
The importance of viscosity of the dispersed phase as compared to the matrix phase was applied in U.S. Pat. No. 5,844,031 which teaches the importance of mixing at a temperature which is within 30xc2x0 C. of a temperature where the difference between the dispersed phase viscosity and the matrix viscosity is the lowest at a predetermined shear rate. That patent, however, does not teach confining fillers to one of the phases to modify its viscosity and processing the compositions at temperatures and shear rates such that the viscosity ratio between the dispersed and the matrix phase is optimized. The patent also does not recognize that this phenomenon can be extended to other dispersed systems wherein the viscosity of a dispersed phase of non-silicone elastomers can also be controlled by adding a controlled amount of the filler.
Drop break-up in the dispersion of polymer blends was first studied by Taylor (G. I. Taylor, Proc. Roy. Soc., A138, 41 (1932); G. I. Taylor, Proc. Roy. Soc., A146, 501 (1934)). Taylor modeled the drop size using the viscosity ratio and the capillary number. For simple shear forces, Taylor balanced the interfacial forces and shear forces to obtain a relationship for the maximum drop size that will be stable. Several researchers have studied the drop break-up phenomenon under various complex conditions highlighting the importance of viscosity ratio in controlling the break-up of the dispersed phase in immiscible polymer blends. A correlation relating capillary number to viscosity ratio in twin screw extruded polymer blends has been given by Wu (S. Wu, Polym. Eng. Sci., 27, 335 (1987)):   D  =            4      ⁢              Γη        r                  ±          0.84                                    γ        .            ⁢              η        m            
where the plus sign in the exponent applies for xcex7r greater than 1 and the minus sign in the exponent applies for xcex7r less than 1. xcex93 is the interfacial tension, {dot over (xcex3)} is the shear rate, D is the diameter of the drop, where xcex7r=xcex7d/xcex7m where xcex7d is the viscosity of the dispersed phase and xcex7m is the viscosity of the matrix phase. This relationship highlights the importance of interfacial tension, viscosity ratio and the matrix phase viscosity in controlling the maximum droplet size of the dispersed phase.
Among the several methods described above to produce stable polymer blends, an efficient method has yet to be developed which does not used compatibilizing techniques, grafting techniques, or block copolymer additives. The importance of the confinement of the filler in the dispersed phase and the importance of viscosity modification with respect to the dispersed phase has yet to be addressed. In addition, delamination continues to be a problem. Therefore, there remains a need for efficient alternative strategies for stabilizing the dispersed phase morphology of immiscible polymer blends in order to prevent delamination and to optimize other blend properties.
In one aspect, the present invention provides a polymer blend which comprises:
a) a matrix phase polymer;
b) a dispersed phase polymer being contained within the matrix phase polymer and initially having a lower viscosity than the matrix phase polymer; and
c) a filler material contained within the dispersed phase polymer to form a modified dispersed phase polymer wherein the filler is substantially contained within the phase boundary of the modified dispersed phase polymer;
wherein the viscosity of the modified dispersed phase polymer is increased by the filler, thereby improving the stability of the dispersion of the modified dispersed phase polymer in the matrix phase polymer.
In another embodiment, the present invention provides a method for the formation of a blend of a matrix phase polymer and a dispersed phase polymer initially having a lower viscosity than the matrix phase polymer, said method comprising:
a) dispersing a filler material within the dispersed phase polymer to form a modified dispersed phase polymer having an increased viscosity wherein the filler is substantially contained within the phase boundary of the dispersed phase polymer; and
b) dispersing the modified dispersed phase polymer within the matrix phase polymer.
In a further embodiment, the present invention provides a method for the formation of a blend of a matrix phase polymer and a dispersed phase polymer which comprises at least one of the following steps:
(I) pre-dispersing a dispersed phase polymer with a filler to form a modified dispersed phase polymer;
(II) mixing in a reactor the modified dispersed phase polymer with a matrix phase polymer to form a powder wherein the temperature in the reactor is less than the melting point of the matrix phase polymer; and
(III) intimately mixing the powder wherein the temperature is greater than the melting point of the matrix phase polymer.